Assessment of the Yellowfin Sole Stock in the Bering Sea and Aleutian Islands Thomas K

Assessment of the yellowfin sole stock in the Bering Sea and Aleutian Islands Thomas K. Wilderbuer, Daniel G. Nichol and James Ianelli Executive Summary Summary of Changes in Assessment Inputs Changes to the input data 1) 2016 fishery age composition. 2) 2016 survey age composition. 3) 2017 trawl survey biomass point estimate and standard error. 4) Estimate of the discarded and retained portions of the 2015 catch. 5) Estimate of total catch made through the end of 2017. Catch of 150,000 t assumed for 2018 and 2019 projection. Changes to the assessment methodology No changes were made to this assessment. Summary of Results The assessment updates last year’s with results and management quantities that are higher than the 2016 assessment primarily due to 1) including 2016 fishery weight-at-age information, 2) including two more years in the spawner-recruit time-series, and 3) a lower estimated survey catchability. Yellowfin sole continue to be well-above BMSY and the annual harvest remains below the ABC level. The female spawning stock is in a slow downward trend. As estimated or As estimated or specified last year for: recommended this year for: Quantity 2017 2018 2018 2019 M (natural mortality rate) 0.12 0.12 0.12 0.12 Tier 1a 1a 1a 1a Projected total (age 6+) biomass (t) 2,290,000 2,202,300 2,553,100 2,460,700 Female spawning biomass (t) 778,600 770,900 895,000 890,000 Projected B0 1,202,700 1,204,000 BMSY 424,000 456,000 FOFL 0.125 0.125 0.12 0.12 maxFABC 0.114 0.114 0.109 0.109 FABC 0.114 0.114 0.109 0.109 OFL (t) 287,000 276,000 306,700 295,600 maxABC (t) 260,800 250,800 277,500 267,500 ABC (t) 260,800 250,800 277,500 267,500 As determined last year for: As determined this year for: Status 2015 2016 2016 2017 Overfishing No n/a No n/a Overfished n/a No n/a No Approaching overfished n/a No n/a No Projections are based on estimated catches of 150,000 t used in place of maximum ABC for 2018 and 2019. Responses to SSC and Plan Team Comments on Assessments in General General comments for all assessments: SSC encourages assessment authors to adopt the current model naming convention. This assessment now complies with the desired naming convention. Responses to SSC and Plan Team Comments Specific to this Assessment The yellowfin sole stock assessment has set an example for its inclusion of ecosystem factors in the assessment. Temperature was found to be related to survey catchability and growth. Interestingly, after conducting a field experiment to further examine relationships between temperature and catchability, it was found that survey biomass is more strongly correlated to wave height than temperature, which is in turn confounded with temperature. The SSC supports the approach outlined in the SAFE to further elucidate the effects of sea state and/or bottom temperature on q, noting that these covariates may act on the assessment in different ways (i.e., availability versus gear efficiency). If an effect of wave height on catchability is confirmed, implying that rougher seas adversely affect the ability of the trawl to tend the seafloor, then this would beg the question whether other assessed species are similarly affected. So, the outcome of this work has the potential for important, far-reaching implications. The results of last summer's sampling efficiency experiment indicated that sea-state had a much larger effect on yellowfin sole herding, and trawl efficiency in general, than bottom temperature. Dan Nichol looked at the time-series of eastern Bering Sea yellowfin sole biomass estimates and found that they also were strongly correlated to wave height. To confuse things more, wave height was correlated to temperature. 1. The experimental results were analyzed and a paper on wave height effect on yellowfin sole catchability was submitted to Fisheries Bulletin (The effects of wave-induced vessel motion on the geometry of a bottom survey trawl and the herding of yellowfin sole by Somerton, Weinberg, Munro, Rugolo and Wilderbuer, In Review). However, this still does not answer the question of why you find a strong temperature effect on the survey catchability function in the stock assessment model. One possibility is that trawl sampling efficiency is more influenced by waves but yellowfin sole availability to the survey is more influenced by temperature. 2. Dan Nichol has completed a CPUE versus waves analysis back to 2005. Although RACE surveys have recorded sea state data for many years it was not entered into the database earlier than 2005, so Dan will do some hand entry, then repeat the analysis with more years and considering wave height and temperature together in a model. This work is viewed as secondary to the availability research (see #3), given that availability likely has a much greater influence on biomass estimates than sea- state/temperature. 3. Dan Nichol also has made progress toward showing that yellowfin sole availability changes with temperature using sex ratio, based on the notion that males arrive first and leave the spawning areas later than females, and that the timing of annual spawning is temperature-dependent. 4. A CIE review is scheduled for May 2018 for yellowfin sole, northern rock sole and Alaska plaice. We will solicit comments from the expert panel on exploring an additional waves parameter to fit the survey biomass. One ongoing issue with the yellowfin sole assessment is a strong retrospective pattern illustrated by Figure 4.21. More recent assessments tend to yield higher estimates of female spawning biomass. Attempts to determine the cause have been unsuccessful so far. The PT suggested that the author consider examining the potential effects of q and M on the retrospective patterns. The SSC encourages ongoing efforts to understand this phenomenon so that appropriate model adjustments can be made. The SSC also recommends updating weight-at-age with each assessment as new data become available annually. Finally, the SSC looks forward to the use of the adopted model naming convention in next year’s yellowfin sole assessment. The retrospective comment is addressed in this assessment under the retrospective analysis heading. Weight-at-age is routinely updated, both for the fishery and the survey. Introduction The yellowfin sole (Limanda aspera) is one of the most abundant flatfish species in the eastern Bering Sea (EBS) and currently is the target of the largest flatfish fishery in the world. They inhabit the EBS shelf and are considered one stock. Abundance in the Aleutian Islands region is negligible. Yellowfin sole are distributed in North American waters from off British Columbia, Canada, (approx. lat. 49o N) to the Chukchi Sea (about lat. 70o N) and south along the Asian coast to about lat. 35o N off the South Korean coast in the Sea of Japan. Adults exhibit a benthic lifestyle and occupy separate winter, spawning and summertime feeding distributions on the eastern Bering Sea shelf. From over-winter grounds near the shelf margins, adults begin a migration onto the inner shelf in April or early May each year for spawning and feeding. The directed fishery has typically occurred from winter through autumn (Wilderbuer et al. 1992). Yellowfin sole are managed as a single stock in the BSAI management area as there is presently no evidence of stock structure. Fishery Yellowfin sole have annually been caught with bottom trawls on the Bering Sea shelf since the fishery began in 1954 and were overexploited by foreign fisheries in 1959-62 when catches averaged 404,000 t annually (Fig. 4.1, top panel). As a result of reduced stock abundance, catches declined to an annual average of 117,800 t from 1963-71 and further declined to an annual average of 50,700 t from 1972-77. The lower yield in this latter period was partially due to the discontinuation of the U.S.S.R. fishery. In the early 1980s, after the stock condition had improved, catches again increased reaching a peak of over 227,000 t in 1985. During the 1980s, there was also a major transition in the characteristics of the fishery. Yellowfin sole were traditionally taken exclusively by foreign fisheries and these fisheries continued to dominate through 1984. However, U.S. fisheries developed rapidly during the 1980s in the form of joint ventures, and during the last half of the decade began to dominate and then take all of the catch as the foreign fisheries were phased out of the EBS. Since 1990, only domestic harvesting and processing has occurred. The management of the yellowfin sole fishery changed significantly in 2008 with the implementation of Amendment 80 to the BSAI Fisheries Management Plan. The Amendment directly allocated fishery resources among BSAI trawl harvesters in consideration of their historic harvest patterns and future harvest needs in order to improve retention and utilization of fishery resources by the non-AFA trawl catcher/processor fleet. This was accomplished by extending the groundfish retention standards to all H&G vessels and also by providing the ability to form cooperatives within the newly formed Amendment 80 sector. In addition, Amendment 80 also mandated additional monitoring requirements which included observer coverage on all hauls, motion-compensating scales for weighing samples, flow scales to obtain accurate catch weight estimates for the entire catch, no mixing of hauls and no on-deck sorting. The partitioning of TAC and PSC (prohibited species catch) among cooperatives has significantly changed the way the annual catch has accumulated (Fig 4.1, bottom panel) and the rate of target catch per bycatch ton.

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